Light distance

By
Ed Rosenthal
on May 15, 2002

How far above my plants should I put my 400 watt High Pressure Sodium lamp. How many plants will it accommodate?
Dave,
Internet

The height of the 400 watt lamp from the top of the plant canopy depends on the light and how it is used. Lamps with air-cooled reflectors can get closer to the canopy than reflectors without a heat barrier. Moving lights can be placed closer than stationary ones.

A stationary unprotected reflector needs a distance of 12 to 15 inches to dissipate the heat generated by the lamp. There are two sources of heat.

The first is the air that has been heated and is being forced down the sides of the reflector. Once this hot air escapes the sides of the reflector, it rises away from the garden. The size of the hot air layer below the reflector is affected by the size of the reflector and the movement of the air current. Usually it extends no more than six inches. However, this air heats the room. Air-cooled reflectors eliminate this heated air so that it doesn’t affect the canopy or the room temperature.

The second source of hot air is infrared light. This is light in a spectrum below red and is invisible to humans. It is emitted by heat lamps. When it hits a solid object it turns into heat so it is especially damaging to plant tissue.

Stationary lights shine continuously on the leaf tissue. Moving lights are intense but then give the leaf respite and the tissue time to cool before it receives another burst of bright light and heat. For this reason, the light can be placed closer than one that doesn’t move.

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